(i) Field of the Invention
The present invention relates to a solid laser source in the field of optoelectronics and a laser application device using the solid laser source, particularly to reduction of noise in an output of a second-harmonic generator.
(ii) Description of the Related Art
With the progress of the highly information-oriented age, in the field of optical recording such as optical disk devices and laser printers, for improving the recording density or for meeting the requirements of high-speed printing, needs of laser source of shorter wavelength rise. But, in a blue range (wavelength of 400 to 480 nm) wherein there are many needs at practical product level, gas laser sources such as He--Cd (helium--cadmium) lasers and Ar (argon) lasers are only put to practical use. When a gas laser source is put in an optical disk device or the like, though the recording density can be considerably improved because of a short wavelength, since the size of the laser source is larger than that of the device for being equipped with it and the power consumption becomes great, putting it to practical use is hindered. But, though there is an example in which a gas laser source is put in a certain kind of laser printer, it is a limited kind for a special application. In view of such conditions, it is of urgent necessity to downsize the short-wavelength laser source and lower the power consumption of it.
Well, since the laser oscillation in the blue range is very difficult in a solid laser as described above, an optical second-harmonic generation (hereinafter called SHG for short) method using a nonlinear optical crystal as a method for obtaining a short wavelength not directly but indirectly is proposed and developed for practical use. In an SHG system, at least a solid laser crystal and a nonlinear optical crystal are disposed in an oscillator composed of a pair of mirrors, a base wave of a long wavelength is first generated by exciting the solid laser crystal from the exterior of the oscillator, and a second harmonic of the base wave is next generated by the nonlinear optical crystal. Besides, because the performance of SHG is closely connected with the characteristics of a semiconductor laser as an exciting light source, it is in a relation that the study for improvement of SHG is advanced always after increasing the power of the semiconductor laser or improving the high stability or the like of it. But, with improvement of the performance of the semiconductor laser, the SHG system has focussed the spotlight of attention. Putting the advantages of the above-described SHG in order, (1) it can be constructed in a small size; (2) a low power consumption can be realized; (3) a high stability of the SHG output by solidity can be intended; and (4) a long duration becomes possible.
As a solid laser source by which a light in the blue range is obtained, there is an internal oscillator type SHG system as shown in FIG. 13 for example.
In FIG. 13, an exciting light 31 from a semiconductor laser (not shown) is introduced into a solid laser crystal 4 to excite the solid laser crystal 4. The excited solid laser crystal emits a specific light according to its composition. This is a base wave light. Accordingly, the wavelength or range of the base wave is determined by the material of the solid laser crystal. But, although there is arbitrariness in material, since the kinds of solid laser crystals are not so much, the range of wavelength to select is necessarily limited. For example, when YAG (Nd:Y.sub.3 Al.sub.5 O.sub.12) is used as the solid laser crystal, a base wave of approximately circular polarization is obtained in a fairly narrow range of wavelength with the center of 1064 nm, while, in an LiSAF crystal described later, it is a base wave of nearly linear polarization with a considerably wide range of wavelength of 750 to 1000 nm. Further, the excited base wave light is introduced into a nonlinear optical crystal (SHG crystal) 6. In the nonlinear optical crystal, a wavelength that meets the phase-matching conditions determined by the relation between the refractive index and the length of optical path in each crystal axis, namely, a second harmonic (SHG output) 33 is emitted. The first and second laser mirrors 3 and 7 constituting an oscillator are given the following characteristics of wavelength selection. That is, the first laser mirror 3 allows the exciting light 31 to pass but reflects a base wave beam 32 and the SHG output 33. On the other hand, the second laser mirror 7 reflects the base wave beam 32 but is made to have a good transmission characteristic for the SHG output 33. In short, the oscillator has the construction by which, while the base wave is shut up, the generated second harmonic is taken out to the exterior of the oscillator. By this construction, mixing of an outer disturbance such as a reflected return light from the exterior of the oscillator can be restrained without providing an optical isolator on the emission side, as a result, the influence of the outer disturbance on the oscillation wavelength of the base wave can be made small and there is another merit related to a stable oscillation.
Well, as disclosed in U.S. Pat. No. 4,811,349, a laser device in which an LiSAF (Cr:LiSrAlF.sub.6 ; fluorolithium-strontium-aluminum with addition of chromium) crystal that oscillates in a wide range of wavelength of 750 to 1000 nm is used as a solid laser crystal is proposed. By using this LiSAF, by which the band of oscillation wavelength becomes remarkably broad in comparison with a conventional crystal, as the above-described internal oscillator type solid laser crystal, it becomes possible selectively to obtain a short wavelength of 375 to 500 nm and the possibility of a variable-wavelength laser source becomes open.
The present inventors have been at grips with development of an SHG light source of blue range as the second oscillation wave obtained by a nonlinear optical crystal wherein an LiSAF crystal is applied to such a semiconductor laser-exciting system as shown in FIG. 13 and an excited laser light is used as the first oscillation wave (base wave), for many years. After this, although the application scope of the SHG light does nothing but extend, in particular, needs to improve the performance of the SHG light source applied to a precisely measuring instrument become intensive more and more, and the present invention is to open the way for a solution to the stability or reduced noise of the output light. The noise in the SHG system is a low-frequency side component of 3 MHz or less in the output light. It was found that increase of noise of low frequency in the SHG laser light has a serious influence from sides of stability or accuracy of the device. Hereinafter, the mechanism of generating noise in a prior art will be described in detail.
FIG. 14 shows a construction of an SHG system in which an LiSAF crystal is applied as a solid laser crystal. Although it is basically the same construction as that in FIG. 13, a wavelength selection element 5 is provided in front of the SHG crystal 6 in the oscillator, and an SHG output 33 of a proper wavelength can be selected by this wavelength selection element 5. An exciting light 31 emitted from the semiconductor laser 11 passes through a convergent optical system 12 and the first laser mirror 3 and then is gathered in the solid laser crystal 4 of an LiSAF crystal to excite the solid laser crystal 4. Further, base wave beams 32A of required wavelengths among base wave beams 32 emitted from the solid laser crystal pass through the wavelength selection element 5 to be incident on the SHG crystal 6. A part of the base wave beams 32A is converted into an SHG light by the SHG crystal and the major part reaches the second laser mirror 7 and is reflected. Because mirror films of dielectric multilayer films for reflecting 99% or more of the base wave beams 32A are formed on the first and second laser mirrors 3 and 7, the base wave beams 32A repeat going and returning in the oscillator. While the base wave beams 32A go and return in the oscillator, emission of SHG output 33 is steadily urged by the SHG crystal 6.
As described above, by combining a solid laser capable of oscillation in a wide band and a wavelength selection element, a blue laser in the range of 375 to 500 nm can be obtained as an SHG output, and the application scope is remarkably extended. Etalon or a birefringent filter, etc. can be utilized as the wavelength selection element. FIG. 14 shows a case of a birefringent filter. The birefringent filter is constructed using a birefringent crystal such as a crystal board and a wavelength is selected by being inclined at the Brewster angle with respect to the incident beam and rotating around the normal axis z shown in the drawing.
In the above-described prior art, however, the SHG light obtained as the output contains noise of 3 MHz or less. Although many interests have been taken in the reduction or restraint of noise from way back, and measures were done, there was no effective proposal of measure. Till the present, the longitudinal mode of the base wave beam oscillated in the oscillator has been considered a cause of noise generation because it brings about multi-oscillation. Qualitatively, the following description is given. In a solid laser of internal oscillator type, because a plurality of base wave longitudinal modes are present in the oscillator at the same time, the intensities vary as the modes interfere in one another. Therefore, intensity conflicts are induced among the base wave modes and it is led in a multi-oscillation state. As a result, it is considered that noise of 3 MHz or less is brought about on the SHG output. (T. Baer, "Large-amplitude fluctuations due to longitudinal mode copling in diode-pumped intracavity-doubled Nd:YAG lasers," J. Opt. Soc. Am. B3,1175 1986).
But, from the description of the above-described physical phenomenon, for intending to reduce noise, a conclusion that a single mode state must be made and kept in the oscillator is introduced. As an actual problem, even if the single mode state is obtained, it is a considerably difficult technique to maintain the single mode with compensating changes in the external environment such as temperature and atmospheric pressure. Therefore, returning to the root to seize the essence of the problem, solution and study of the mechanism of noise generation were made again, and, based on a quite different conception from methods considered conventionally, solution of the problem of the prior art was tried and the noise reduction was successful. Since the construction of the present invention is basically the same as the prior art, although the presence of the invention is not clear in appearance, merit is in that the relation between optical parts disposed in an oscillator and the length of the oscillator is distinctly regulated. Because this is an application of an optical fundamental principle, there is universality that it is applicable to all cases of SHG lasers using solid lasers.